Axis Labels, Numeric Labels, or Both? Line Graph Styles to Consider

Data visualization is more about strategic thinking than about following steadfast rules.
Take a simple line graph, for example.
How will you label your line graph?
With vertical axis labels and light gray grid lines? With labels directly above or on top of the data points? A mix of both?
Here are four styles to consider.

Option A: Label the vertical axis

The first option is to simply label your vertical y-axis: 0, 25, 50, 75, 100, and so on.
The trick is strike a balance between labeling too frequently and not frequently enough. In this fictional scenario, I used increments of 25. The increments you choose will likely depend on your unique dataset.
Then, lighten (mute) the grid lines. Thin gray lines > thick black lines. We need our viewers to focus on the star of the show — the burgundy and orange lines — and not get sidetracked by the backup dancers — the supplemental information like grid lines and tick marks.
I use this style when I want viewers to focus on the general, big-picture view. Is the line generally going up or going down? Where are the peaks and valleys over time?
The viewers won’t see the exact values. In other words, my spreadsheet will tell me that Organization A had a 130 in 2009. But my viewers can only estimate that value.
The viewers’ takeaway message might be, “Organization A’s values are always above Organization B’s values. Both organizations have higher numbers in 2015 compared to 2009. Organization A started around 125 and went up to the 175-200 range, and Organization B started in the 25-50 range, got as high as the 100-125 range, but then went back down to the 75-100 range. And what the heck happened to Organization B between 2014 and 2015?”
Sometimes I add markers (those little circles on top of the lines).
I include markers when I want my viewers to remember that each point represents a different point in time. Rather than the smoothed-out appearance in the line above, this style subtly emphasizes that there gains and losses over time. Make sure your markers are relatively small; otherwise, the graph can look outdated and clunky.

Option B: Label all of the data points directly

A second option is to remove the axis and label the data points directly. Direct labeling means placing the labels as close to the data as possible. In this case, the numeric labels go right above, or on top of, the data points. We’re aiming for physical proximity.
You might choose to place the labels directly above the lines. However, this style tends to get a bit cluttered, especially when there are more than two lines per graph, or if you have lots of points in time to display.
To avoid some clutter, I often center the numeric labels directly on top of each data point:
Or, you might center the numeric labels directly on top of circular markers.
Meh.
The circles need to be pretty large to fit two-digit and three-digit labels. And if my labels included percentage signs, then the circles would need to be even larger.
This style gets clunky fast. It reminds me of something I would draw in elementary school. Feel free to disagree… I don’t have research to back this up. It’s just my personal aesthetic preference.

Option D: Label just a couple points along the line

Finally, a fourth option is to only label a few points along the line.
You might label the beginning and end points. Or, you might label a specific year or two. For example, you might be telling a story about what happened in 2012 specifically. If so, you could label the 2012 point only.
This style helps you avoid information overload and is often preferred among laypeople viewers who want the big-picture, birds-eye-view of information. If your viewers are researchers or data scientists who love seeing alllll the raw data, I wouldn’t recommend this style.
You might forego the vertical axis labels:
Or, you might include the vertical axis labels: